Claw, take the weekend to gather your thoughts and de-stress if possible. Your head must be full of 100s of thoughts & emotions just now.
If you can find a Solicitor that deals in CP Law & Social Work Practise, that would be most useful for you. IPSEA etc. deal with Education Law, so cannot advise on this.
The assessment procedure you have mentioned several times, is an assessment for a dc with ASN-SN, identified by any agency, including a parent, and passed to the Child & Families Disability Team. To move this situation forward, you must find out, if your ds has been referred to this team. (Of course, dc with ASN can have CP issues too), & you need to establish whether ds has been referred due to his ASN, or because of suspected CP concerns.
The school, have passed on their "concerns" to SS. At the moment, your ds, according to the SW you spoke to, does Not meet the threshold to begin CP proceedings. But, be clear, the SW is saying the Multi-agency meeting will help her to decide, if after hearing all agencies opinions and of course yours, whether there is now enough evidence for CP threshold to be met. Until this meeting, it is unlikely that SS will close the file.
I'm trying to help you make sense of what is happening at present, as reading your posts, it appears you are, understandably, a bit confused.
So, in the time you have before the Meeting, take Legal advise, prepare a File with Reports from CAMHS, etc, that explain ds difficulties.
School may be "causing trouble", but you don't want to be ambushed at the meeting, & make sure you take someone with you.
No-one is going to take your ds away, & hopefully the meeting will open the door for SS to provide support for ds.
After the meeting, it will be cleare if you need to put a hold on information-sharing between Agencies, & whether to take your ds out of school etc.
Take Care & try not to worry too much.